The exhaust of internal combustion engines often contains substances of which the release into the environment is undesirable. Therefore, many countries set limits which have to be adhered to on the release of these pollutants, such as in the exhausts of industrial facilities or automobiles. These pollutants include nitrogen oxides (NOx), such as in particular nitrogen monoxide (NO) or nitrogen dioxide (NO2), as well as a range of other pollutants.
The release of these nitrogen oxides from the exhaust of combustion engines can be reduced in various ways. At this point, emphasis should be placed on reduction by way of additional exhaust treatment measures, in particular based on selective catalytic reduction (SCR). What these methods have in common is that a reducing agent which acts selectively on the nitrogen oxides is added to the exhaust, the nitrogen oxides thus being converted in the presence of a corresponding catalyst (SCR catalyst). This converts the nitrogen oxides into substances which are less harmful to the environment, such as nitrogen and water.
One reducing agent for nitrogen oxides which is already used nowadays is urea (H2N—CO—NH2), which is added to the exhaust in the form of an aqueous urea solution. In this context, the urea in the exhaust stream may break down into ammonia (NH3), for example as a result of the action of heat (thermolysis) and/or a reaction with water (hydrolysis). The ammonia which is thus formed is the actual reducing agent for nitrogen oxides.
Exhaust treatment systems for automobiles have been being developed for some time, and this is discussed in numerous publications. Thus for example European Patent EP 487 886 B1 discloses a method for selective catalytic NOx reduction in oxygen-containing exhaust of diesel engines, in which urea and the thermolysis products thereof are used as reducing agents. In addition, a device for generating ammonia in the form of a tubular evaporator is disclosed, and comprises a spraying device, an evaporator comprising an evaporator tube, and a hydrolysis catalyst.
Further, European patent EP 1 052 009 B1 discloses a method and a device for carrying out the method for thermal hydrolysis and metering of urea or urea solutions in a reactor with the aid of a partial exhaust stream. In the method, a partial stream of the exhaust is removed from an exhaust line upstream from the SCR catalyst and passed through the reactor, the partial stream, which is loaded with ammonia after the hydrolysis in the reactor, likewise further being passed back into the exhaust line again upstream from the SCR catalyst.
In addition, European patent EP 1 338 562 B1 discloses a device and method which make use of the catalytic reduction of nitrogen oxides by ammonia. In this context, the ammonia is obtained from urea in solid form under flash thermolysis conditions and from isocyanic acid by hydrolysis, and supplied to the exhaust stream of a vehicle.
Further, European patent application EP 1 348 840 A1 discloses an exhaust purification system in the form of an assembly, which can be transported as a whole unit, in the form of a 20-foot container. The system is operated in such a way that a urea or ammonia solution is injected into the exhaust stream directly by means of an injection device. The nitrogen oxides contained in the exhaust are reduced on an SCR catalyst.
Further, German patent application DE 10 2006 023 147 A1 discloses a device for generating ammonia which is part of an exhaust treatment system.
In addition, international applications WO 2008/077 587 A1 and WO 2008/077 588 A1 disclose a method for the selective catalytic reduction of nitrogen oxides in exhausts of vehicles by means of aqueous guanidinium salt solutions. This method uses a reactor which generates ammonia from the aqueous guanidinium salt solutions.
Even though ammonia gas generators have been known for some time, thus far the technology has not been implemented in a vehicle or any other application. Thus far, the concept of direct injection of an ammonia precursor substance into the exhaust stream of an internal combustion engine has been pursued, this ammonia precursor substance being broken down into the actual reducing agent in the exhaust stream by suitable measures. However, as a result of incomplete decomposition or secondary reactions of decomposition products in the exhaust line, depositions are always observed, and damage the catalysts and filters which are also present in the exhaust line.
Therefore, an object of the present invention is to provide an ammonia gas generator which overcomes these drawbacks of the prior art. A further object of the present invention is to provide an ammonia gas generator which is of a simple construction, provides a high conversion rate of ammonia precursor substances into ammonia gas, and makes long-term use without maintenance possible. In addition, it should be possible to use the ammonia gas generator universally, it also being possible in particular to use different types of ammonia precursor substances.